A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In so called “immersion” lithography, the substrate is immersed in the lithographic projection apparatus in a liquid having a relatively high refractive index, an immersion liquid. For example, the immersion liquid may be water. The immersion liquid fills a space between the projection system and the substrate. In this way, imaging of smaller features is enabled because the exposure radiation has a shorter wavelength in the liquid.
Thus, immersion systems have water or another liquid between the projection system and the substrate during exposure instead of only air or other gas. This results in a direct and strong influence of a thermal variation of the liquid on imaging, since the refractive index of the liquid is a function of temperature. Also, because of strong thermal coupling of the immersion fluid temperature to the projection system, and in particular the optical element of the projection system closest to the substrate, any thermal variation of the liquid will likely have an effect on the imaging properties of the total projection system. Typical thermal control of the immersion liquid may not be good enough to be negligible in terms of focus and imaging related overlay (magnification and distortion). This may be an issue in particular, during scanning exposures where temperature cycles are observed for each substrate. Further, it may become progressively more difficult and more expensive to dynamically control the temperature of the liquid to the required degree with decreasing critical dimension (CD) of the structures being imaged.